Lithographic processes generally employ at least two steps to prepare a plate for use in a printing press, namely 1) exposure of a plate coated with a photosensitive composition to a light source to create a photoimage and 2) a subsequent development step. Typically, the development step involves washing away material from the plate with the aid of a developer solution which may be highly alkaline (e.g., pH of 13), and/or based on an organic solvent. Alternatively, heating has sometimes been employed to effect development of the photoimage.
Both of the above-described development processes can suffer from the drawbacks of being relatively time-consuming and expensive. Furthermore, when volatile organic or strongly alkaline developer solutions are used, their disposal presents an environmental problem.
What is needed in the industry are photosensitive compositions which do not have the above-described drawbacks.
A variety of different materials have been used in photosensitive compositions in the past. Photosensitive compositions employing polymers with pendant acid sensitive moieties have been described in the art (Ito, H.; Ueda, M. Macromolecules 1988, 21, 1475-82). o-Nitrobenzyl substituted polyacrylates have been described in U.S. Pat. No. 3,849,137. t-Butyl substituted polyacrylates have also been reported (Ito, H.; Willson, C. G. in Proc. SPIE-Int. Soc. Opt. Eng. 1987, 771, 24; and U.S. Pat. No. 4,491,628).
U.S. Pat. No. 4,963,463 claims a radiation sensitive resin composition comprising an alkali-soluble resin, an o-quinone diazide, an acid-sensitive esters of nitrobenzyl or cyanobenzyl alcohol.
End-capped polyphthalaldehyde has been employed in imaging systems in combination with 'onium salts which were used as a source of photogenerated acid (Ito, H.; Willson, C. G. Polym. Eng. Sci. 1983, 23, 1013). Photoresists based on thermal polycarbonate degradation in the presence of photogenerated acid have also been described (Frechet, J. M. J.; Bouchard, F.; Houlihan, F. M.; Kryczka, B.; Eichler, E.; Clecak, N.; Willson, C. G. J. Imag. Sci. 1986, 30, 59). Both of these systems function by cleavage of the polymer backbone.
Canadian Patent No. 672,947 describes protective films comprising copolymers of tetrahydropyran-2-yl acrylates, and glycidyl esters of acrylates. Those films are thermally processed, thereby effecting cleavage of the tetrahydropyran-2-yl groups, and subsequently causing cross-linking of the carboxylic acid and the epoxy residues.
Benzyl, benzhydryl, and triphenylmethyl acrylates have been imaged using high energy radiation (e.g., electron beam, X-ray, and ion beam) sources followed by development with aqueous alkaline solutions as described in Japanese Kokai applications 59-075244, and 58-068743.
Dimethylbenzyl methacrylates have been employed in combination with iodonium salts as deep-UV photoresists (Ito, H. Polym. Mater. Sci. Eng. 1989, 60, 142).
.alpha.-Substituted benzyl methacrylate polymers have been photoimaged and developed with an alkaline developer (Ito, H.; Ueda, M.; Ebina, M. ACS Symp. Ser. 1989, 412, 57-73).
Japanese Kokai applications 63-256492 and 3-317388 describe several direct-image lithographic plate formulations employing polymers having side-chain groups which, following an etching development step, cleave to form hydroxyl and carboxyl groups.
Japanese Kokai patent applications 63-094691 and 63-100192 describe lithographic plates containing polymers with acid labile groups including some alkoxyalkyl esters. The plates also contain cross-linking agents which appear to cross-link with carboxylic acid residues as they are formed. The plates appear to act in a negative-tone and may involve a development step.
Japanese Kokai application 62-299,313 describes the use of polymers containing acid anhydride residues in combination with o-nitrobenzyl esters of carboxylic acids. Those polymers are sensitive only to UV radiation.
U.S. Pat. No. 5,102,771 discloses photosensitive compositions comprising: (a) a photoinitiator which generates an acid upon exposure to radiation; and (b) a polymer having acid labile groups pendant from the polymer backbone. Those materials were combined with maleic anhydride polymers to provide increased adhesion. However, the resulting photosensitive compositions have only a moderate shelf-life under ambient conditions.
The photosensitive compositions of the present inventions require no wash off or thermal development step have superior shelf-life and press life and comparable speed to the prior compositions.
U.S. Pat. No. 5,077,174 (Bauer et al.) also discloses a positive working resist composition which comprises a polymer having acid-labile pendant groups of a defined formula.